Sheer trouble with unknown dissidence (6)
I believe the answer is:
heresy
'dissidence' is the definition.
(thesaurus)
'sheer trouble with unknown' is the wordplay.
'trouble with' indicates an anagram.
'unknown' becomes 'y' (algebra).
'sheer' mixed up with 'y' gives 'HERESY'.
(Other definitions for heresy that I've seen before include "Opinion contrary to orthodox belief" , "Deviation from orthodox religious thinking" , "Religious non-conformity" , "Belief that's anathema to the orthodox" , "you shouldn't believe it" .)